Composite materials typically consist of a matrix in which high strength and modulus fibers are embedded for added modulus, strength or toughness. The fiber-matrix interface in composite materials plays a central role in determining the performance of a given composite, especially in metal and ceramic matrix composites. The strength and toughness of a composite material is influenced by the strength and fracture energy of the bond between the fiber and the matrix as well as the friction between the fiber and matrix once the bond is broken. The magnitude of the friction force between the fiber and the matrix is dependent upon the residual clamping stresses (compressive stresses) exerted on the fiber by the surrounding matrix, the coefficient of friction between the fiber and the matrix, and the area over which the fiber is in contact with matrix.
Currently, the interface properties of composite materials is typically determined by conducting tests on single fiber composite test specimens. One common single fiber composite interface test technique is the fiber pullout test in which a single fiber of a composite test specimen is pulled axially from the surrounding matrix. The force required to debond the fiber from the matrix as well as the force required to displace the fiber relative to the matrix after debonding is measured. Data from this test is difficult to interpret due to non-linear variations of stress, strain and frictional tractions along the embedded fiber length. These non-linear stress and strain variations arise primarily from a narrowing of the diameter of the fiber along its length caused by the lateral contraction of the fiber and which is otherwise known as Poisson's ratio effect. This results in variable compression forces exerted by the matrix on the fiber along the length of the embedded fiber which in turn causes variations of frictional force along the fiber-matrix interface. As a result, it is difficult to assign a simple frictional traction value to the fiber-matrix interface.
Another common single fiber test technique is the fiber pushout test which is similar to the fiber pullout test except that a fiber of the composite is pushed axially into the matrix. Data from this test is also difficult to interpret due to Poisson's ratio effect which, in this case, causes the diameter of the fiber to expand rather than contract. In addition, the data in the pushout test is also influenced by edge effects. Typically, edge effects are caused by defects such as cracks in the matrix, fiber or interface at both ends of the test specimen which weaken the interface. Edge effects are more dominant in the pushout test than the pullout test because a short specimen length is required for conducting the pushout test. Edge effects in short specimens represent a high percentage of the matrix along the embedded fiber length and as a result, further distorts fiber-matrix interface data in pushout test specimens. Pushout and pullout tests on identical specimens render dramatically different results, presumably due to the Poisson effect influencing respective test data in opposite ways.